Knee osteoarthritis (OA) is a chronic degenerative disease that affects approximately one third of all older adults in the United States. The joint damage and chronic pain accompanying the progression of knee OA compromises quality of life and physical function and facilitates the onset of physical disability in older adults. Evidence from several randomized controlled trials demonstrates that physical activity interventions result in significant improvements in physical function and quality of life among seniors with knee OA. Despite the benefits accompanying exercise, promoting long-term maintenance of physical activity participation among older knee OA patients remains problematic. One potential explanation for the excessive attrition from structured physical activity interventions is that traditional approaches to exercise therapy fail to provide adequate instruction or practice in the self-regulatory skills necessary to maintain independent, long-term participation. Therefore, the primary goal of this investigation is to determine if a group-mediated cognitive behavioral (GMCB) physical activity intervention involving individualized and group-based activity counseling will result in higher weekly frequency and volume of moderate intensity or greater physical activity (PARMod+) at 3 months and 12 month follow-up assessments when compared to a traditional physical activity intervention. The GMCB intervention program incorporates group and individual behavior change activity counseling to help patients develop self-regulatory skills that facilitate the maintenance of independent physical activity participation while concomitantly phasing out center-based activity over an initial 3-month period. It is our hypothesis that the GMCB intervention program will produce significantly higher frequency and volume of PARMod+ participation at the end of 3 and 12 months when compared to a traditional physical activity program. Secondary aims of the investigation are to determine the impact of the GMCB and traditional physical activity interventions on self-reported physical function and pain, mobility performance, and health-related quality of life. Public Health Relevance Statement: The proposed investigation examines the efficacy of behavioral interventions for promoting long-term maintenance of regular physical activity and enhancing disease symptoms in older adults with knee OA. Given that OA is one of the most prevalent diseases affecting older American and is considered one of the primary causes of disability with aging, the application addresses an important public health issue that will only grow in magnitude as the aging of the U.S. population continues to increase.